Processes to convert renewable resources into transportation fuels usually involve several steps. For example, one approach is to use acids to convert carbohydrates, starches, lignins, and other biomass into sugars such as glucose, lactose, fructose, sucrose, dextrose. The catalytic hydrogenation of the carbonyl groups of a sugar like glucose (C6H12O6) can then produce a polyalcohol including sorbitol (C6H12O6).
There has been a significant effort to produce lower polyols through catalytic hydrotreating of aqueous sorbitol. Various Group VIII metal hydrotreating catalysts have been discussed including nickel (U.S. Pat. No. 4,338,472), ruthenium (U.S. Pat. No. 4,496,780, U.S. Pat. No. 6,291,725), and rhenium (U.S. Pat. No. 6,479,713, U.S. Pat. No. 6,841,085). Alditols including 15-40 wt % sorbitol solution in water are catalytically hydrocracked between 400° to 500° F. and hydrogen partial pressure from 1200 to 2000 psig in a fixed bed catalytic reactor using nickel catalyst to produce at least 30 wt % conversion to glycerol and glycol products (U.S. Pat. No. 4,338,472). An alkali promoter such as calcium hydroxide or sodium hydroxide was added to the feedstream solution to control pH, prevent nickel leaching and enhance conversion. Sorbitol was hydrocracked over a supported Group VIII noble metal catalyst with an alkaline earth metal oxide; such ruthenium on a titanium alumina support with barium oxide between 300° to 480° F. at 500 to 5000 psig to produce lower polyols such as glycerol, ethylene glycol, 1,2-propanediol (U.S. Pat. No. 4,496,780). High molecular weight polyols including sugar alcohols such as sorbitol or xylitol in water with a base promoter underwent hydrogenolysis over a metal catalyst of ruthenium deposited on an alumina, titania, or carbon support between 350° to 480° F. at 500 to 2000 psig hydrogen to produce low molecular weight polyols including glycerol, propylene glycol, and ethylene glycol (U.S. Pat. No. 6,291,725). Five carbon sugars and sugar alcohols including 15-40 wt % sorbitol, and lactic acid were hydrocracked with hydrogen over a rhenium catalyst in water to achieve at least 30 wt % conversion to glycerol and glycol products between 400° and 500° F., between 1200 and 2000 psig hydrogen, and a liquid hourly space velocity of 1.5 to 3.0 (U.S. Pat. No. 6,479,713). Battelle (2005) reacts an aqueous solution of sorbitol with hydrogen over a multi-metallic rhenium catalyst, including Re and Ni, at 250°-375° F. to produce propylene glycol through hydrogenolysis of C—O and C—C bonds (U.S. Pat. No. 6,841,085). These methods are limited by size, temperature, products, and conversion rates. Unfortunately at higher temperatures and higher catalytic activity, these reactions become quickly fouled. The catalyst must be removed and replaced before sufficient volumes of fuel are processed. Thus, these reactions must be improved to meet a commercial production scale and cost effectiveness.
Some advances have been made toward the catalytic conversion of sorbitol to alkanes. Huber, et al., (2004) used Palladium, Silica, and Alumina catalysts to convert sorbitol to a stream of alkanes including butane, pentane, and hexane. Incorporating hydrogenation of reaction intermediates with produced hydrogen increased yield. David, et al. (2004) assayed conditions for the production of hydrogen and/or alkanes from renewable feeds including aqueous solutions of sorbitol. In a review, Metzger (2006) notes alkane production from aqueous phase sorbitol reforming is improved with a bi-functional catalyst including a metal (Pt, Pd, or the like) and acid including silica alumina with the co-production of H2 and CO2. Although the yield of alkanes could be increased up to 98% when hydrogen was co-fed with the aqueous sorbitol stream they were able to reduce CO2 production, increasing H2O production and pathway efficiency.
Many of the processes above do not remove oxygen, require expensive catalysts, are subject to fouling, and are not scalable to production levels required. Additionally, processing biomass as a common feedstock is hindered by short catalyst lifetime, increased pressures and temperatures, increased production of coke byproducts, and increased corrosiveness. These undesirable side-effects hinder mass production of renewable fuels from biomass. Although noble metals have been used for hydrotreating at lower temperatures, these expensive catalysts do not alleviate the problem of fouling and the reactions are difficult to perform on a commercial scale. A method of converting large quantities of biomass is required that does not damage catalysts and equipment during the refining process.